BUDAPEST, DEC. 5 -- President Clinton and Russian President Boris Yeltsin drew sharply opposing pictures of Europe's future today, with Yeltsin warning that plans Clinton supports for an expansion of NATO threaten to make an enemy of Russia.

The verbal clash highlighted a day of stark contrasts at the opening meeting of the Conference on Security and Cooperation in Europe, a loose 53-member grouping that aspires to play a larger role on the continent now that the Cold War is over. With the CSCE as a backdrop, the United States, Russia and Ukraine finalized arms agreements to reduce the chances of nuclear conflict and agreed that Europe should not again be split into hostile camps. But their glad tidings were shadowed by the brutal warfare in Bosnia and the sharp U.S.-Russian differences over security for Eastern Europe's former Soviet allies.

Clinton and Yeltsin displayed clear disagreement on the role NATO ought to play in coming years for those countries. In particular, Yeltsin denounced plans, which NATO approved only last week, to prepare for extension of its security guarantees to some former Soviet satellites.

"Europe, not having yet freed itself from the heritage of the Cold War, is in danger of plunging into a cold peace," Yeltsin said. "Why sow the seeds of mistrust? After all, we are no longer enemies. We are all partners."

The Russians have voiced irritation at the main implication of expanding the North Atlantic Treaty Organization, which is that Russia remains a threat. Unpersuaded by surface arguments that general stability in Europe is the goal, Moscow has complained that in fact renewed Russian expansionism is what East Europeans fear -- and what Western governments have begun to worry about as well with the rise of extreme nationalists in Russian politics over the last year.

Yeltsin's voice betrayed hints of bitterness as he indirectly condemned the United States' role in the moves to expand the U.S.-led defense pact, which he said risks isolating Russia. "It is a dangerous delusion to suppose that the destinies of continents and the world community in general can somehow be managed from one single capital," he said.

"We hear explanations to the effect that this is allegedly the expansion of stability -- just in case there are undesirable developments in Russia," he said. "If on those grounds ... the intentions are to move the responsibilities of NATO up to Russia's borders, let me say one thing: It's too early to bury democracy in Russia."

Clinton spoke just before Yeltsin and described NATO as "the bedrock of security in Europe." He said "no country outside will be allowed to veto expansion" -- a clear reference to Russia.

As if anticipating Yeltsin's objections, Clinton said NATO is no threat. "As NATO expands, so will security for all European states, for it is not an aggressive, but an offensive organization," he said, apparently meaning to say defensive. "NATO's new members, old members and nonmembers alike will be more secure."

The confrontational aspects of the two speeches marked a pause, at least, in the amiable Bill and Boris series of meetings dating from Vancouver early last year through Rome, Tokyo, Moscow and finally Washington in September. Clinton, during his quick six-hour visit here, made no effort to meet formally with Yeltsin, although they chatted very briefly on Bosnia, NATO and the nuclear agreements.

U.S. officials played down Yeltsin's harsh words by characterizing them as a sop to critics back home in Moscow. They say that privately, Yeltsin has told Clinton that he does not object to expansion, but only to quick expansion.

The officials expect Russia to take part in the Partnership for Peace, a program of military cooperation that will prepare former Warsaw Pact countries for membership in NATO. Russia had thought the partnership, which sets no specific criteria for joining NATO, was as far as the Atlantic alliance was going to go in the foreseeable future.

However, during the next year, NATO plans to set minimum requirements for membership, making the partnership a matter of secondary importance. Last week at a NATO meeting, Russian Foreign Minister Andrei Kozyrev balked at signing Moscow up for partnership activities.

The Clinton-Yeltsin rhetorical face-off here was an odd prelude to the successful finalization of major nuclear reduction treaties. Ukraine formalized its agreement to the nuclear Non-Proliferation Treaty, which in turn brought into force the 1991 Start I treaty, negotiated between Washington and Moscow.

Ukraine's signature was necessary because it inherited nuclear weapons from the Soviet Union and has not yet given them up. Last January, Ukraine agreed to surrender its weapons to Moscow, and through the Non-Proliferation Treaty to become formally nuclear-free state. Two other Soviet republics, Kazakhstan and Belarus, already had agreed to give up their weapons, after the United States and Britain promised to consult with the former Soviet republics if they were threatened with nuclear war.

Late Sunday night, the Ukraine agreement almost fell through. Russia was nervous that Kiev might try to keep its remaining weapons because the Ukrainian parliament recently insisted the weapons belong to Ukraine.

Secretary of State Warren Christopher rushed to see Kozyrev at the Hungarian parliament building, and the two drew up a side agreement between Moscow and Kiev that said that no matter what Ukraine's parliament said, Ukraine would give up the warheads. Ukrainian officials later accepted this provision, U.S. officials said.

The agreements mean the United States and Russia can proceed on ratification of the START II treaty, which would bring deeper cuts in nuclear arsenals. Start I eliminates 9,000 warheads held by the United States and Russia. Start II would eliminate another 5,000, leaving each side with 3,500.

The overall atmosphere at various meeting halls was confused by images of the war in Bosnia playing on television monitors alongside broadcasts of CSCE proceedings.

Bosnian President Alija Izetbegovic spoke angrily to the group, blaming the West for the warfare in his country. "What is happening in Bosnia is no more than the weakness of the West," he said. "The result will be a discredited United Nations, a ruined NATO and a Europe demoralized... . There will be different, a worse world."

He ridiculed the U.N. peacekeeping effort in the Balkans, led by Britain and France, for its focus solely on supplying food and medicine. "Against a serious illness, they apply tranquilizers," he said.

Having already agreed to a near-even partition of Bosnia with the separatist Serbs, Izetbegovic said his government would make no more concessions.

Today, Clinton noted that the war rages only 300 miles from Budapest and called on the Serbs to settle by renegotiating a peace plan developed last spring. He discussed ways to prevent future such wars, including upgrading the CSCE into a peacekeeping group.

This issue also divided Washington and Moscow.

When it was founded in 1975, CSCE was a bridge between East and West. Since the end of the Cold War, it has dabbled in mediation and continues to monitor human rights abuses. Now, it is looking for something new to do.

U.S. officials say the CSCE is a place where countries that fall outside NATO can feel secure. The Americans want to change the name from conference to organization and let it organize missions to stop ethnic violence in Central and Eastern Europe.

"The CSCE should be our first flexible line of defense against ethnic and regional conflicts," Clinton said.

Russia, on the other hand, sees NATO and Moscow's own Commonwealth of Independent States -- a loose successor organization to the Soviet Union -- as subordinate to the CSCE. Such an arrangement would in effect give Russia a say in NATO activities.